Suspect the ring on top that holds the pump and sending unit in. Anything else would leak while filling it up or while driving. The next thing would be the vapor hose that goes to the charcoal canister under the driver's seat area.

The clue that the ring on top of the tank is loose or the gasket is mispositioned is when you fill up and the gas pump shuts off automatically, if you wait a few seconds, you'll be able to pump in another half gallon or more. That will keep happening each time you do that because there is about a six to eight gallon air space on top so the tank can compress in a crash. As the air leaks out, a little more gas can be forced in. If you do that long enough you'll have gas running out onto the ground.

If that is not the case, suspect the vapor hoses. You can start with a visual inspection but the best approach is to use a "smoke machine". It puts out a white non-toxic smoke at 2 psi. You inject that somewhere in the hose and watch for where the smoke leaks out.